1. Field of the Invention
The present invention relates to a calibrating device for calibration of an image measurement system using a camera. Specifically, the present invention relates to a calibrating device imaged by a camera for calibrating the internal parameters of the camera, mechanical parameters of a robot, etc., and an image measurement system comprising the calibrating device.
2. Description of the Related Art
Conventionally, an image measurement system using a camera is utilized in various industrial field. In such an image measurement system, an object is imaged by a camera, the image is subjected to image processing by an image processing apparatus, etc., and the position, shape, size, and angle of the object are measured. Further, in order to use a camera for measurement, an operator must calibrate (correction) the camera using a calibrating device prior to the measurement. Usually, in an image measurement system, the characteristic portion (including arbitrary graphics and patterns, the same below) provided on the calibrating device is imaged by a camera, the outline, position, etc., of the characteristic portion is detected from the picked-up measurement image, and calibration is performed based on the detection result. As a conventional calibration technology, for example, non-patent document “Roger Y. Tsai, “An Efficient and Accurate Camera Calibration Technique for 3D Machine Vision”, Proceedings of IEEE Conference on Computer Vision and Pattern Recognition, Miami Beach, Fla., 1986, pages 364-374” discloses a method for calibrating internal parameters (lens focal distance, lens distortion, lens center, etc., of a camera) of a camera using a calibrating device. Further, Japanese Patent No. 3394278 discloses a method for setting a sensor coordinate system of a visual sensor including a camera using a calibrating device.
However, recently, an image measurement system using a camera has also been used for a measurement apparatus for calibrating a robot system. For example, Japanese Unexamined Patent Publication (Kokai) No. 2008-12604 discloses a measurement apparatus for calibrating mechanical parameters of a robot using a camera. Japanese Patent No. 4021413 discloses a measurement apparatus for measuring a to-be-measured target using a light receiving device (camera) mounted near the tip of a robot arm. Japanese Unexamined Patent Publication (Kokai) No. 2005-300230 discloses a measurement apparatus for measuring the position of TCP (Tool Center Point) of a robot using a camera.
When calibration is performed using a camera as mentioned above, accuracy of the shape or the position of the characteristic portion provided on the calibrating device, and the robustness of the illumination conditions have direct influence on the calibration results. For example, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2005-300230, in order to accurately measure the position of the Tool Center Point, a mark, which is a characteristic portion, must be accurately arranged or formed on the Tool Center Point of the robot and the base of the tool.
Conventionally, the characteristic portion was formed by printing or applying a colored paint to the calibrating device. However, it was difficult to accurately print or apply color paint to the calibrating device. Further, when the characteristic portion is formed by applying colored paint, the surface of the paint may be smooth depending on the forming method, and the characteristic portion reflects the outside light, and thus, the image measurement system cannot stably detect the characteristic portion. Also, the paint may be altered by temperature or may be melted by an organic solvent, and further, there are various limitations in the use of the paint in some places. Therefore, when the characteristic portion is formed by paint, such limitations in the use of the paint must be taken into consideration.
Thus, various calibrating devices have been developed in order to precisely arrange or form the characteristic portion on the calibrating device. For example, Japanese Patent No. 3409931 discloses a magnification calibration plate comprising a transparent substrate such as glass, etc., on which a deposition material, such as chrome, is vapor-deposited as a pattern which serves as a characteristic portion. Japanese Patent No. 3477139 discloses a calibration gauge comprising a metal plate on which a line segment which serves as a characteristic portion is drawn with colored ink. Japanese Patent No. 3512092 discloses a calibration apparatus wherein a design having a predetermined pattern which serves as a characteristic portion is displayed using a liquid crystal panel. Japanese Patent No. 3575165 discloses a calibrating device provided with a positioning member having a recess and a completely spherical mark body which is held in the recess of the positioning member. Japanese Patent No. 3737919 discloses a photogrammetric target comprising a plurality of reflective sheets imaged at a relatively high brilliance and adhered on the measurement portion, and non-reflective sheets provided around the reflective sheets.
However, the above-mentioned conventional methods have the following drawbacks. In the case of the magnification calibration plate disclosed in Japanese Patent No. 3409931, it is difficult to accurately vapor-deposit deposition materials such as chrome. Also, in the case of the calibration gauge disclosed in Japanese Patent No. 3477139, it is difficult to accurately draw the line segment on the metal plate with colored ink. In addition, similar to applying paint, the surface of the colored ink may be smooth and reflect outside light. Therefore, there is a problem that depending on illumination conditions, the characteristic portion cannot be distinguished from the surroundings due to the reflected light, and thus, the image measurement system cannot stably detect the characteristic portion.
The calibration apparatus disclosed in Japanese Patent No. 3512092 has a problem that an expensive liquid crystal panel is required, which leads to an increase in production costs. In the calibrating device disclosed in Japanese Patent No. 3575165, there is a problem that the spherical mark which serves as a characteristic portion must be produced with accuracy, which leads to an increase in the production costs. In the photogrammetric target disclosed in Japanese Patent No. 3737919, it is difficult to accurately adhere the plurality of reflective sheets.